There is little doubt that we are in the midst of a worldwide epidemic of diabetes. Almost 16 million people in the US are thought to be afflicted, a third of whom are undiagnosed. Insulin resistance is recognized as a characteristic trait of the disease, defined by the inability to respond to normal circulating levels of insulin. The primary lesion in this state involves defects in the uptake and storage of glucose in muscle and fat cells. Targeting these defects holds the key to the development of new therapeutic approaches. However, understanding the specific lesions that cause insulin resistance in patients with type 2 diabetes will first require a better grasp of the cell biology of insulin action. To this end, the molecular events involved in the regulation of glycogen synthesis by insulin will be investigated, with special attention to the role of a newly described scaffolding protein, PTG, in the regulation of protein dephosphorylation. In Aim l, the molecular basis for the existence of multiple scaffolding proteins for protein phosphatase 1 (PP1) will be investigated, evaluating the hypothesis that each member of this family has a defined separate function in signal transduction. The structure/function relationships of these molecules will be analyzed by a series of deletion and chimeric mutants. This will be followed by evaluation of the effects of the mutants on glycogen metabolism in cell models, and in intact rat liver. Aim 2 will focus on dissecting the signaling pathway that mediates the activation of PP1 by insulin at the glycogen pellet, resulting in the stimulation of glycogen synthase. This approach will pursue two novel hypotheses involving the identification of regulatory proteins that might be substrates for phosphorylation. The physiological function of PTG will be investigated in Aim 3, by the targeted disruption of the PTG gene in mice. Together, these approaches will allow for the evaluation of the importance of this pathway in insulin action, setting the stage for future investigations into its potential role in the development of diabetes.